Cysts can be caused by the clogging of sebaceous glands or by infection, notes WebMD. These closed pockets of tissue become filled with fluid, pus or other material. This makes them feel like peas under the skin.
Cysts are slow-growing, are noncancerous and usually are not painful, unless they become infected, inflamed or ruptured states WebMD. The treatment of cysts involves piercing them with a scalpel so that their contents drain out. Injecting cortisone medication into an inflamed cyst can cause it to shrink. If cysts do not respond to these treatments or reoccur, a doctor may opt to remove them surgically.